Medical devices deliver electrical stimulation in order treat a variety of ailments or symptoms of patients, such as pain, epilepsy, movement disorders, incontinence, sexual dysfunction, gastroparesis, or other neurological, urological or gastric disorders. The medical devices used to treat such ailments or symptoms may be implantable. Further, whether implanted or not, the medical devices often deliver electrical stimulation to targeted tissue via one or more electrodes carried by one or more leads, which include internal conductors to couple the electrodes to the medical device.
For example, spinal cord stimulation (SCS) has been used to treat chronic pain, such as chronic neuropathic pain of the trunk and limbs. Usually, after a percutaneous trial with an external medical device has shown that SCS is efficacious, an implantable medical device is implanted surgically. The external trial device and the implantable medical device both generate electrical pulses, which may be delivered within the spinal canal by selected electrodes from among a plurality of electrodes. The electrodes are carried by one or more implanted multi-electrode leads, which include conductors to couple the electrodes to the devices. Lead extensions with corresponding conductors may be used to couple the leads and lead conductors to the devices. The trial and implantable medical devices may be coupled to the same leads and extensions, or to different leads or extensions.
For SCS, the one or more multi-electrode leads are typically implanted outside of the dura, in the epidural space. When a lead is implanted in the epidural space, the electrodes carried by the lead are usually approximately two to six millimeters away from the targeted neurons of the spinal cord. Between the electrodes and the neurons to be excited are the dura, the arachnoid membrane, and a layer of cerebrospinal fluid. These elements tend to diffuse electrical currents.
At least in part due to the distance and above-identified elements between the epidurally-located electrodes and target neurons, it is difficult to keep the effect of stimulation constant when there is movement of the implanted electrodes relative to the target neurons. For example, by bending forward, a patient can cause epidurally-implanted electrodes to move several centimeters relative to a target spinal cord level or nerve root. Additionally, when a patient goes from a supine lying position, to sitting, to standing, the space between the epidurally-implanted electrodes and the surface of the dorsal columns of the spinal cord can change significantly. Such movement may require adjustment of stimulation parameters, such as amplitude or pulse width, by a factor of two or more to maintain substantially constant stimulation efficacy.
If the targeted tissue is within the cervical levels of the spinal cord, the movement of the implanted electrodes relative to the targeted tissue may be even more significant, e.g., when the neck is turned or tilted. Some patients experience the stimulation as varying within a range from very painful to no sensation at all with relatively minor movements of the head and neck. The difficulty in maintaining substantially constant stimulation efficacy throughout a range of patient motion has limited usage of SCS therapy, particularly in patients with pain in the upper limbs, shoulders or neck.